present invention relates to a heat treatment apparatus. A semiconductor fabrication process includes heat treatment steps, such as film deposition, oxidation, diffusion etc. As a heat treatment apparatus which conducts these treatments there is used a vertical heat treatment apparatus which has the advantage of taking in little air during operation.
As shown in FIG. 16, this conventional heat treating apparatus includes a transfer chamber 12 which is an operation space formed below a vertical heat treatment furnace 11, and a wafer boat 13 which is vertically moved between the transfer chamber 12 and the heat treatment furnace by boat elevator 14. Semiconductor wafers W are mounted on the wafer boat 13 vertically spaced from each other to be carried on the wafer boat 13 into the heat treatment furnace 11 for a heat treatment there. After the heat treatment, the wafer boat 13 is loaded out of the heat treatment furnace 11 to dismount the wafers W from the wafer boat 13.
In this heat treatment apparatus, for preventing particles in the ambient atmosphere and particles generated when the wafers are transferred from sticking to the wafers W, air which has been cleaned through a dust removing filter unit 15 is supplied into the transfer chamber 12 from a circulating ventilation air flue (not shown).
The filter unit 15 comprises a filter material folded in a bellows which is secured to a frame at the top and bottom and at both sides by an adhesive or seal member. Since the wafers unloaded out of the heat treatment furnace have high temperatures, radiant heat from the wafers W heats the filter unit 15 up to high temperatures, and organic components, e.g., hydrocarbon, etc., are scattered from the adhesive, e.g., resin used in the filter unit 15. The organic components react with films deposited on the heated semiconductor wafers W to adversely deteriorate electric characteristics, film performance of the deposited film, etc., with a result of lower yields of the wafers. This is much affective to micronization of wafers for above 16 MDRM.
Filter materials for the filter unit 15 contain impurities, such as boron (B), etc., and when the filter materials are exposed to high temperatures or air having an acid atmosphere, boron, etc. scatter in larger amounts. As patterns of devices are more micronized, the impurities from the filter materials much affect yields of the wafers.
The filters of filter units are commonly provided by ULPA (Ultra Low Penetration Air) filters. Conventionally ULPA filter of this kind are made of borosilicate glass fiber as a filter material, which is based on SiO.sub.2, B.sub.2 O.sub.3, etc.
Recently wafers have been manufactured with larger diameters and, in addition, micronized, deposited film layers, as of silicon oxide films, silicon nitride film, etc., formed by heat treatments, have become thinner. Property control of the deposited film layers, e.g., for electric characteristics are becoming more severe. Qualities of the deposited film layers are much affected by traces of impurities in the ambient atmosphere.
In this connection, it has been found that since the filter material of the conventional ULPA filters is borosilicate glass fiber based on SiO.sub.2, B.sub.2 O.sub.3, etc., when the filter material itself is exposed to bad conditions, such as exposure to acid components used in wafer washing and quartz washing and to radiant heat from the wafer boat and reaction tube immediately after a heat treatment, boron (B) is generated. Consequently there has been a risk that the boron (8) may mix into clean air which has passed the filter material.
The filter material of this kind is generally folded in the bellows of an accordion which is secured to a suitable frame. Adhesives and spacers for securing the filter material are made from resins containing volatile solvents. There is a risk that in the above-mentioned acid atmosphere or high-temperature atmosphere, organic gas may be generated from these adhesives and spacers themselves and contaminate the clean air which has passed through the filter.
Thus the conventional heat treatment apparatus has a risk that boron and organic components generated from the filter itself stay on the surfaces of wafers and contaminate the wafers, with a result of low yields.